This invention relates generally to an electrophotographic printing machine, and more particularly concerns an improved development system for use therein.
In the process of electrostatographic printing, an electrostatic latent image is recorded and reproduced in viewable form. The field of electrostatography includes electrophotography and electrography. Electrophotography employs a photosensitive medium to form, with the aid of electromagnetic radiation, the electrostatic latent image. Electrography utilizes an insulating medium to form, without the aid of electromagnetic radiation, the electrostatic latent image. Various types of reproducing machines have been developed which utilize the foregoing types of electrostatographic printing. Development, which is the act of rendering an electrostatic latent image viewable, is employed in all of the aforementioned classes of electrostatographic printing. Hereinafter, an electrophotographic printing process is described as an illustrative embodiment.
The process of electrophotographic printing, described in U.S. Pat. No. 2,297,691 issued to Carlson in 1942 and further amplified by many related patents in the art, utilizes a photosensitive element having a photoconductive insulating layer which is charged to a uniform potential to sensitize the surface thereof. The charged photoconductive surface is exposed to a light image of an original document being reproduced. As a consequence of the exposure, the charge is selectively dissipated in the irradiated areas in accordance with the light intensity reaching the surface thereof. This records an electrostatic latent image on the photoconductive surface. Development of the electrostatic latent image recorded on the photoconductive surface is achieved by bringing a developer mix into contact therewith. A typical developer mix generally comprises dyed or colored heat-settable plastic powders, known in the art as toner particles, which are mixed with coarser carrier granules, such as ferro-magnetic granules. The toner particles and carrier granules are selected such that the toner particles acquire the appropriate tribo-electric charge relative to the electrostatic latent image recorded on the photoconductive surface. When the developer mix is brought into contact with the charged photoconductive surface, the greater attractive force of the electrostatic latent image recorded thereon causes the toner particles to transfer from the carrier granules and adhere to the electrostatic latent image.
Recently, multi-color electrophotographic printing has been developed. The multi-color printing process is similar to black and white printing. In color electrophotographic printing, the light image is filtered producing successive single color light images of the original document. These colored light images expose the charged photoconductive surface to create successive single color electrostatic latent images thereon. Each single color electrostatic latent image is developed with toner particles complementary in color to the color of the filtered light image. The toner powder images are then transferred from the electrostatic latent image to a sheet of support material, in superimposed registration with one another. This multi-color powder image is then permanently affixed to the sheet of support material forming a color copy thereon.
In multi-color electrophotographic printing, it is highly desirable to utilize a belt for high speed printing. In such a process, the inter-image spacing between successive latent images is critical. Nominally, the inter-image spacing must be equal to the spacing between successive developer units so as to prevent the toner powder image of one color from having toner particles deposited thereon of a different color. It is highly desirable to reduce this inter-image spacing so as to increase the speed of the printing process.
Accordingly, it is a primary object of the present invention to improve the development system of an electrophotographic printing machine by reducing the inter-image spacing.